Extensive use is made of asphalt paving as a means for surfacing for general traffic use, both on primary and secondary roads, as well as parking lots and, in some locations, as sidewalks. Asphalt is a dark brown to black cementitious material in which the predominating constituents are bitumens which occur in nature or are obtained in petroleum processing. The Asphalt Institute considers the term "asphalt" to include asphalt cements, asphalt fluxes, asphalt cutbacks, asphalt emulsions, asphalt road oils, roofing and waterproofing asphalts and all other asphalts and asphalt residuums used in the manufacture of asphalts and asphalt specialties. Such widespread use creates an ongoing demand for repair and preventative maintenance. Over prolonged periods of time, for various reasons, the asphalt surface deteriorates or fails or is otherwise damaged and requires repair. Pavements in need of maintenance or repair can exhibit any or all of the following conditions:
"Raveling" is the progressive separation of aggregate particles in a pavement from the surface downward. Usually, the fine aggregate comes off first and leaves little "pock marks" in the pavement surface. As the process continues, larger and larger particles are broken free, and the pavement soon has the rough and jagged appearance typical of surface erosion. Raveling can result from lack of compaction during construction, construction during wet or cold weather, dirty or disintegrating aggregate, poor mix design, or extrinsic damage to the pavement.
"Shrinkage Cracks" are interconnected cracks forming a series of large blocks, usually with sharp corners or angles. They are caused by volume changes in the asphalt mix, in the base, or in the subgrade. "Alligator Cracks" are interconnected cracks forming a series of small blocks resembling an alligator's skin or chicken-wire. In most cases, alligator cracking is caused by excessive deflection of the surface over unstable subgrade or lower courses of the pavement. The unstable support usually is the result of saturated granular bases or subgrade. The affected areas in most cases are not large; sometimes, however, they will cover entire sections of a pavement, and when this happens, it usually is due to repeated loadings exceeding the load-carrying capacity of the pavement.
"Upheaval" is the localized upward displacement of a pavement due to swelling of the subgrade or some portion of the pavement structure. In colder climates, upheaval is commonly caused by expansion of ice in the lower courses of the pavement or the subgrade. It may also be caused by the swelling effect of moisture on expansive soils.
"Pot Holes" are bowl-shaped holes of various sizes in the pavement, resulting from localized disintegration of the pavement under traffic. Contributory factors can be improper asphalt mix design, insufficient pavement thickness, or poor drainage. Also, pot holes may simply be the result of neglecting other types of pavement distress.
"Grade Depressions" are localized low areas of limited size which may or may not be accompanied by cracking. They may be caused by traffic heavier than that for which the pavement was designed, by settlement of the lower pavement layers, or by poor construction methods.
The major failure of asphalt surfacing results from moisture penetration of the base material. This penetration of moisture is generally caused and/or accelerated by overloading small units of area through numerous repetitive cycles until the asphalt covering disrupts or separates. Once this occurs, of course, moisture seeps into the base material and not only naturally deteriorates the material, but also may freeze and cause separation of large amounts or quantities of material which are reduced to small particles by continued use of the surface thus eventually causing a hole or depressed area commonly called a "pot hole" as set forth earlier.
In order to prevent or minimize this type of damage from occurring, the asphalt surface must be periodically sealed with a seal coat solution that penetrates any separations or disruptions in the asphalt surface thus preventing moisture from entering therein. If, however, the pot hole occurs, the only way to prevent additional damage is to trim and excavate the failed area or pot hole, remove any dust, dirt or excess material, reseal the exposed base to preclude any additional moisture from entering therein and replacing the removed asphalt surface with a cold or hot asphalt mix and tamping, compacting, or rolling the hot or cold mix until it achieves the proper density and elevation with respect to the surrounding asphalt surface.
General repair and maintenance of asphalt paving is usually the responsibility of City, County, State and Federal governmental agencies although numerous private contractors also represent not only themselves but also such governmental agencies and owners of parking lots in shopping centers, apartments, etc., in seal coating asphalt surfaces or repairing damaged such surfaces. The number and size of vehicles and equipment which are needed to repair asphalt surfaces depends, of course, upon the quantity of work to be done; however, even in small jobs with just a few pot holes to repair, the vehicles and equipment generally used include a two-and-a-half ton dump truck with cold mix, a portable air compressor driven by an auxiliary power unit and which is pulled by the dump truck, pneumatic tools such as chisels, hammers, air pressure nozzles, tampers and liquid spraying wands, all of which utilize compressed air generated and stored by the air compressor and its associated tank, a portable, gas-fired emulsion tank or sealing compound solution tank, a one-half ton pickup truck which is used to pull the compressor and miscellaneous hand tools such as brooms, shovels, mops, and the like. In addition, at least one person is required to operate each vehicle and perform the required work. Generally a crew of four individuals are involved in such an asphalt repairing operation.
In general, the entire repair fleet goes to the area having the damaged asphalt surface. The air compressor is driven by an auxiliary power unit and is used to compress air to a usable operating pressure. The gas-fired heat source is started to heat the emulsion liquid in the emulsion tank. The emulsion is a petroleum based liquid that is old and well-known in the art and is used as a sealing-bonding agent to prevent moisture seepage from passing any surface coated with the emulsion. To facilitate its use in a wide variety of applications, in varying climatic conditions and with an extensive range of aggregates, emulsified asphalt is manufactured in several different grades. However, as a general matter, emulsified asphalt is an emulsion of asphalt cement and water that contains a small amount of an emulsifying agent. It is a heterogeneous system containing two normally immiscible phases (asphalt and water) in which the water generally forms the continuous phase of the emulsion, and minute globules of asphalt form the discontinuous phase. Depending on the emulsifying agent, emulsified asphalt may be one of two types: cationic, having electro-positively-charged asphalt globules, or anionic, having electro-negatively-charged asphalt globules.
During the time the emulsion is heating, the crew connects the desired pneumatic tools to the air compressor and trims away the partially failed asphalt surface. For instance, around a pot hole, an area is removed which totally surrounds and is at least one foot from the damaged area and, generally, the trimmed hole would be roughly rectangular in shape. Further, the hole will be to a depth required to remove any damaged or loose materials. In the next step, an air nozzle is connected to the air compressor and high pressure air is used to remove all excess soil and material from the repaired area.
Next the cleaned, prepared area is coated with a light application of a tack coat, an emulsified asphalt diluted with water and otherwise known as the emulsion. It is used to ensure a bond between the surface being repaired and the overlying course of asphalt which consists of mineral aggregate bonded together with the emulsified asphalt. As stated earlier, there are many different types of emulsified asphalts, and presently there are sixteen different grades each designed for a specific use. The repaired area is then filled with a dense-graded asphalt mix which may be either hot mix or cold mix but is generally cold mix for small jobs. The cold mix asphalt consists of mineral aggregate uniformly coated with emulsified asphalt and is old and well-known in the art.
The cold mix asphalt is then shoveled into the hole to be repaired and a pneumatic tamping or compacting tool is attached to the pressurized air source and used to tamp or compact the asphalt tightly into place. If the hole is more than six inches deep, each layer of asphalt applied to the hole is compacted thoroughly so that the cold mix and emulsion are firmly bonded. The hole is built up to grade by the addition of the subsequent emulsion, asphalt layers with enough berm added so that settling and road action will eventually blend the patch with the surrounding asphalt. At this time, the repair crew moves to the next location and the procedure is repeated.
It is readily apparent that the investment in personnel, vehicles and equipment is high relative to the amount of actual work needed to be performed. In addition, much of the equipment is specifically designed to accomplish one job only. For example, the air compressor which is towed by one of the vehicles is simply used to provide compressed air and has no other function. The same thing is true for the emulsion tank and the vehicles where the vehicles usually carry the tools and cold mix and, of course, they tow the emulsion tank and the air compressor. It is obvious, then, that the equipment and personnel required to patch asphalt paving as set forth above involves such expense as to preclude a small business owner such as an apartment owner or shopping center owner from operating and maintaining his own equipment for such asphalt repairs.
The advantage, of course, of using multiple vehicles and separately towed emulsion tanks and air compressors is that the towed items can be unhooked from the vehicles when the job is completed and the vehicle can be used for other functions.